18
F. M. Romero et al.
leaves (Gómez-Lama Cabanás et al. 2014; Schilirò et al. 2012). Another example
of induced resistance was described by Sahu et al. (2019), where three Bacillus
strains from a collection of endophytes from tomato plants were selected due to their
antagonism to Sclerotium rolfsii. These isolates reduced disease incidence up to 67%
under greenhouse conditions (Sahu et al. 2019). Moreover, endophyte-inoculation
reduced the reactive oxygen species (ROS) generated at the site of the S. rolfsii
infection and induced the expression of pathogenesis-related (P R) genes. In this
trend, one of the isolates was able to induce the expression of genes PR1a, PR2b,
and PR3 in absence of the pathogen, while others showed a mild induction of these
genes that was enhanced upon infection with the pathogen. Further, these isolates
were able to induce the activity of defense-related enzymes such as phenylalanine
ammonia-lyase (PAL), peroxidase, polyphenol oxidase, and ascorbic acid oxidase
(Sahu et al. 2019).
Importantly, besides their roles in the synthesis of bioactive molecules, these
enzymes are involved in plant cell wall reinforcement. Thus, it is probable that their
activities can help to prevent the infection by necrotrophic pathogens. This might
explain also the mechanism of protection exerted by two Stenotrophomonas strains
with the ability to colonize Arabidopsis leaves, which modified the expression of
different enzymes involved in cell wall synthesis and reduce lesion sizes provoked
by the necrotrophic pathogens S. sclerotiorum and B. cinerea (Marina et al. 2019).
Accordingly, when cell wall extracts obtained from inoculated leaves were used as
a substrate for pathogens growth on agar plates, there was a reduction in fungal
colony radius compared to plates supplemented with cell wall extracts from mockinoculated leaves. Besides, Stenotrophomonas inoculation induced callose deposition and expression of PR genes associated with the SA and JA signaling pathways.
Similarly, endophytes isolated from Solanum tuberosum able to induce resistance
to Pectobacterium atrosepticum in potato and are able to increase defense-related
enzyme activities both before and after pathogen challenge. Moreover, these isolates
primed the expression of PR genes involved in both, the SA and JA signaling pathways
(Ardanov et al. 2011). The ability to induce the expression of genes involved in phytohormones signaling was also reported by an apoplast-colonizing endophyte from
canola leaves (Romero et al. 2019). This isolate showed antagonism against different
phytopathogens in vitro and planta. The mechanisms proposed to be involved include
the production of antimicrobial compounds as well as the ability to induce defense
mechanisms mediated by SA and JA in the host.
Transcriptional changes induced by beneficial microbes usually differ from the
changes induced by pathogens, mainly in the intensity of induction (Romero et al.
2017). Burkholderia phytofirmans PsJN induced the expression of defense-related
genes in grapevine cell suspension but to a lesser extent than non-host bacterium
P. syringae pv. pisi. Both bacteria-induced medium alkalization, but the endophyte
did not provoke ROS production or cell death, which are observed in pathogentreated cells (Bordiec et al. 2011). This isolate also primed the expression of PR
genes involved in the SA and JA signaling pathways upon pathogen infection in
Arabidopsis, showing more sustained expression of PDF1.2, a JA dependent PR
gene (Su et al. 2017).
F. M. Romero et al.
leaves (Gómez-Lama Cabanás et al. 2014; Schilirò et al. 2012). Another example
of induced resistance was described by Sahu et al. (2019), where three Bacillus
strains from a collection of endophytes from tomato plants were selected due to their
antagonism to Sclerotium rolfsii. These isolates reduced disease incidence up to 67%
under greenhouse conditions (Sahu et al. 2019). Moreover, endophyte-inoculation
reduced the reactive oxygen species (ROS) generated at the site of the S. rolfsii
infection and induced the expression of pathogenesis-related (P R) genes. In this
trend, one of the isolates was able to induce the expression of genes PR1a, PR2b,
and PR3 in absence of the pathogen, while others showed a mild induction of these
genes that was enhanced upon infection with the pathogen. Further, these isolates
were able to induce the activity of defense-related enzymes such as phenylalanine
ammonia-lyase (PAL), peroxidase, polyphenol oxidase, and ascorbic acid oxidase
(Sahu et al. 2019).
Importantly, besides their roles in the synthesis of bioactive molecules, these
enzymes are involved in plant cell wall reinforcement. Thus, it is probable that their
activities can help to prevent the infection by necrotrophic pathogens. This might
explain also the mechanism of protection exerted by two Stenotrophomonas strains
with the ability to colonize Arabidopsis leaves, which modified the expression of
different enzymes involved in cell wall synthesis and reduce lesion sizes provoked
by the necrotrophic pathogens S. sclerotiorum and B. cinerea (Marina et al. 2019).
Accordingly, when cell wall extracts obtained from inoculated leaves were used as
a substrate for pathogens growth on agar plates, there was a reduction in fungal
colony radius compared to plates supplemented with cell wall extracts from mockinoculated leaves. Besides, Stenotrophomonas inoculation induced callose deposition and expression of PR genes associated with the SA and JA signaling pathways.
Similarly, endophytes isolated from Solanum tuberosum able to induce resistance
to Pectobacterium atrosepticum in potato and are able to increase defense-related
enzyme activities both before and after pathogen challenge. Moreover, these isolates
primed the expression of PR genes involved in both, the SA and JA signaling pathways
(Ardanov et al. 2011). The ability to induce the expression of genes involved in phytohormones signaling was also reported by an apoplast-colonizing endophyte from
canola leaves (Romero et al. 2019). This isolate showed antagonism against different
phytopathogens in vitro and planta. The mechanisms proposed to be involved include
the production of antimicrobial compounds as well as the ability to induce defense
mechanisms mediated by SA and JA in the host.
Transcriptional changes induced by beneficial microbes usually differ from the
changes induced by pathogens, mainly in the intensity of induction (Romero et al.
2017). Burkholderia phytofirmans PsJN induced the expression of defense-related
genes in grapevine cell suspension but to a lesser extent than non-host bacterium
P. syringae pv. pisi. Both bacteria-induced medium alkalization, but the endophyte
did not provoke ROS production or cell death, which are observed in pathogentreated cells (Bordiec et al. 2011). This isolate also primed the expression of PR
genes involved in the SA and JA signaling pathways upon pathogen infection in
Arabidopsis, showing more sustained expression of PDF1.2, a JA dependent PR
gene (Su et al. 2017).